Shifter bosses are two studs which are attached to the downtube of a bicycle frame. They serve as mounts for the shifter levers that are used to actuate the derailleurs during gear shifts.
The conventional way of mounting the shifter levers on the downtube of a bicycle frame is to braze mounting bosses onto the downtube. With steel bicycle frames, brazing is relatively simple and provides a durable attachment without significant diminishment of the mechanical properties of the steel tubing. Although generally reliable, brazing does have drawbacks. It requires skilled labor. Jigs and fixtures are required to hold the bosses on the tube. Extensive surface preparation is required to insure clean surfaces and a strong brazed joint. The heat required to braze weakens the tube. In the event the brazed joint or the boss itself breaks, the only way to repair the frame is to braze on a new boss. This requires repainting and the cost is high.
In the case of aluminum frames, which are becoming increasingly popular and in more widespread use, brazing of the shifter mounting bosses and other fittings to the aluminum tubes is impractical, if not impossible. Aluminum brazing alloys have melting temperatures that are relatively close to the melting point of the base metal and it is tricky, at best, to use torches to braze fittings on the aluminum tubes. Brazing in furnaces or molten salt baths is prohibitively expensive and causes various difficulties as far as the effect on mechanical properties of the aluminum and cleaning the end product are concerned. Welding the fittings to the aluminum tubing also presents problems, not only because of the effect of welding on the mechanical properties of the tubing and the difficulty of obtaining good quality welds between the tubing and the relatively small pieces involved but also because it is impractical to use machined bosses and die-cast bosses cannot be welded due to their porous structure.
The assignee of the present invention has had reasonably good results by using rivots and special adhesives to fasten the shifter bosses to the downtube. Unfortunately, however, the incidence of failure of these joints has been undesirably high.
An object of the present invention is to provide a simple, low-cost, reliable attachment between a bicycle frame downtube and shifter lever bosses. A further object is to provide such an attachment in a form that allows field repair and replacement.